Born as seventh of eleven children in Chico, a city in Butte County, California, Koscki started to work as stuntman in the entertainment industry in the early 1980s. He is probably best known for doubling actor Mike Myers in all three Austin Powers films, Austin Powers: International Man of Mystery (1997), Austin Powers: The Spy Who Shagged Me (1999), and Austin Powers in Goldmember (2002). He also doubled Myers in the 2003 fantasy film Dr. Seuss' The Cat in the Hat. Other actors he doubled include Richard Dreyfuss in the crime drama Mad Dog Time (1996), Rick Moranis in the comic adaptation The Flintstones (1994) and the sport comedy Little Giants (1994), William H. Macy in the crime movie On the Dead Side (1995) and the thriller Edmond (2005), Martin Short in the comedy sequel Father of the Bride Part II (1995), the comedy Jiminy Glick in Lalawood (2004), and the sequel The Santa Clause 3: The Escape Clause (2006), David Spade in the comedy Lost & Found (1999) and the television series Just Shoot Me! (2001), and Thomas Lennon in the comedy Reno 911!: Miami (2007).

Koscki trained car stunt driving at Rick Seaman's Hollywood Stunt Driving School until 1992. From 2006 on, he then worked as Driving Instructor himself at this school. He also trained stunt rigging under Lane Leavitt and was specialized in motorcycle stunt work.

After he worked as assistant stunt coordinator on projects such as the romance Under the Boardwalk (1989, working with Dan Koko) and the comedy Think Big (1989, with George Colucci and Tommy J. Huff), Koscki became stunt coordinator himself on projects including the science fiction film Timebomb (1991, with Tracy Scoggins, Irving E. Lewis, and Kurt D. Lott), the horror film Project Vampire (1993), the horror comedy Head of the Family (1996), the science fiction film Murdercycle (1999), the thriller Net Games (2003, with Monica Staggs), the television series Happy Family (2003-2004, starring John Larroquette and Susan Gibney), the horror film Reeker (2005, with Oliver Keller and Nancy Thurston) and its sequel No Man's Land: The Rise of Reeker (2008, with William Devital), the horror thriller Neighborhood Watch (2005), the horror film Night of the Living Dead 3D (2006), the comedy series Sonny with a Chance (2009-2010), the comedy Bad Actress (2011), and the comedy Camp Fred (2012).

More recently, he worked as stunt coordinator on the horror thriller Preservation (2014, stunts by Danny Downey), on the first season of the television series The Thundermans (2013-2014), on the television comedy Terry the Tomboy (2014), and on the short horror film Boarding House (2014), and performed stunts in True Blood (2014, with Craig Baxley, Jr., Theo Kypri, Erik Stabenau, Mark Aaron Wagner, and stunt coordination by Hiro Koda). He also completed filming as stunt coordinator on the drama The Grounds (2014, with Michael Welch) and on the comedy Loaded (2015).

Koscki was still filming the science fiction film Wizardream as stunt and fight coordinator (with Malcolm McDowell and Raymond Forchion) when he went out for a private motorcycle ride on Sunday 5 October 2014. He was brought to hospital after he suffered a cardiac arrest and went into a coma. He passed away on Thursday, 9 October 2014, survived by his wife and his two daughters. [2][3]